SunOvens recommends dark, thin walled pans with lids for best results, but any pan works. Shiny pans reflect the sunlight instead of absorbing it, lowering the temperature. Cast iron will take a little longer to heat up, but will also retain the heat longer, so would work fine.

The cooking chamber will also only fit certain sizes of pans. a standard cookie sheet will be too big, but I did fit a 9×13 pan in with the long side going front to back in the oven, so maybe that gives you some reference as to the size of the chamber. Hope that helps some :)